Inasmuch as the manufacture of ground wood pulp has been known for over 100 years, a detailed discussion of the manufacturing procedure is not necessary. Briefly, grindstones are used to grind logs into wood pulp. During this grinding operation, the grindstones gradually become worn, and, therefore, they must be dressed or sharpened. The degree of sharpness affects both the characteristics of the pulp and, of course, the power consumption of the motor which drives the grindstones.
Heretofore, it has been the task of the operating personnel to decide when the various grindstones in a grinding mill should be sharpened and how to formulate a strategy for sharpening the grindstones. Since this is a rather difficult and complicated task, in view of the pulp quality and the power consumption per ton of pulp, as well as the output per hour, being affected in a rather complicated manner which is not completely understood, objective criteria are desired to aid the operating personnel in making these decisions.
In view of the present demanding requirements for good and reproducible wood pulp quality (the strength of newsprint, for example, in modern rotary presses is an essential, limiting factor for the printing speed), it has become extremely important to be able to maintain good process control in wood pulp mills, so that uniform and dependable quality can be achieved. The production of uniform wood pulp quality and hence uniform paper quality is especially important in the production of newsprint, inasmuch as newspaper presses must be set for the lowest strength newsprint which might ever be run. Thus, the printing speed of such presses depends largely on the lowest strength newspring which might be run, rather than the average strength which may be quite high. By manufacturing newsprint of a more uniform quality, higher printing speeds can be utilized.
The problem of monitoring the gradual change in the sharpness of grindstones in wood pulp mills has been addressed in the past. A device is disclosed in Swiss Patent Specification 151 691 in which the wood feed is measured at a constant feeding pressure and motor power consumption, thereby deriving a measure of the sharpness of the grindstone. The grinder disclosed in the Swiss Patent Specification is of the Stetig-Schleifer type and has only a single continuously operating pocket, which operates by way of a servo system so that the load on the motor driving the grindstone remains constant.
However, measuring the degree of sharpness of grindstones in a Great Northern type grinder, which has two different grindstones, each with two pockets, coupled by a common shaft to a single motor, is significantly more complicated than measuring the degree of sharpness of a grindstone of a Stetig-Schleifer type grinder. A conceivable method using the device disclosed in the Swiss Patent Specification would be to shut off the feed to the pockets of one of the grindstones and then measure the feed pressure and the load on the motor when only one of the grindstones is in operation. Such a theoretical measuring operation is, however, virtually impossible to carry out during operation of a Great Northern type grinder, since it would involve, among other things, the motor being driven at lower power, often at less than half power, thereby giving rise to control problems, or increasing the feed pressure, thereby producing a different and inferior quality pulp during the measuring operation. Accordingly, such a theoretical measuring operation is impractical when continuous production is required.
In order to improve the operating conditions and achieve a better and more uniform pulp quality, the degree of sharpness of the grindstones should be continuously, or almost continuously, monitored during their continuous operation. This would facilitate the formulation of grindstone sharpening strategies.
Basically, there are two important factors to consider when developing a strategy for sharpening grindstones. These are: (i) high capacity and (ii) optimum use of the available power of the motor.
During operation, the grindstones become worn, i.e., less sharp. A freshly dressed stone has fewer abrasive particles in operation. All other conditions being the same, this means a reduced load on the motor and a lower output per unit of time, but, on the other hand, a lower energy consumption per ton of pulp produced. Although the latter result is advantageous per se, the time delay for dressing and the problem of achieving a uniform quality when starting up after dressing present an optimization problem as to when re-dressing is to be done.
It is known and described in, for example, the article on pages 409-411 in Svensk Papperstidning by J. Bergstrom et al., entitled "Analysis of Grinding Process Variables", No. 11, June 15, 1957, that the output of a grindstone is proportional to the square of the power. The factor of proportionality varies with the degree of sharpness of the stone. This factor of proportionality is designated S. If the power is designated P, and if there is selected as a measure of output the rate of wood fed down against the grindstone and call this variable h, the following generally valid equation is obtained: EQU P.sup.2 =h/S (1)
This equation has been confirmed by various investigations, both by the assignee of this application at Ortvikens Trasliperi in Sundsvall and in a larger foreign investigation, the so-called Camel project, reported by D. K. Alexander in Paper Trade Journal, Aug. 9, 1979, p. 26. These investigations establish that the exponent in equation (1) is close to 2 with minor variations. This corresponds quite well with results from grinding in general.
With reference to FIG. 1 of the drawings, there is shown a graph which illustrates how output or production varies with variations in the sharpness of a grindstone, as represented by the proportionality or stone-wood factor (S). More particularly, the perpendicular axes show the production in tons per grinding day and the sharpness or stone-wood factor (S) in arbitrary units, respectively. Further, there are illustrated two families of curves, namely, solid line curves for specific energy consumption (MW/tons) and broken line curves for motor power (MW). The curves for three different process strategies are also illustrated, namely, for keeping constant energy per produced ton of pulp, constant production (tons per grinding day) and constant power. It can be seen that starting with a sharpened stone in the first-mentioned strategy, i.e., constant energy, productivity gradually increases with increased power requirements as the stone-wood factor (S) decreases, i.e., as the grindstone becomes worn. Further, it can be seen that if production is kept contant, both power requirements and specific energy consumption increase as the stone-wood factor (S) decreases. If constant power is to be drawn, it can be seen that production decreases and specific energy consumption increases as the stone-wood factor (S) decreases.
As indicated above, the quality of the pulp obtained is dependent on the grinding conditions. However, no direct measurement of the quality of the pulp is possible during operation. Rather, what must be resorted to are measurements of freeness. These are made by a well-known standard method designated CSF (Canadian Standard Freeness), in which measurements are taken directly on the fiber slurry obtained as a result of the grinding operation. Although what one is primarily interested in is actually the quality of the paper which is to be made, all experience shows that control to a constant CSF value provides entirely adequate paper quality control, since the tearing resistance correlates well with the CSF value.
Of the control principles for grinders which have been suggested, namely, constant piston pressure, constant rate of feed and constant power, tests have shown that the most advantageous for uniform quality is constant rate of feed. The advantage lies in the fact that a newly dressed stone, which otherwise has a tendency to produce coarse pulp, does not produce such coarse pulp when the feed rate is maintained constant, thus producing the most uniform pulp. However, the power consumed at the end of the period between two successive stone dressing operations is relatively large. If more than one grinder is in operation so that the pulp produced is a product of all of the grinders, it has been found that the feed rates need not be strictly maintained at constant values.
It has also been found, and is actually the basic principle for operation of grinders of Great Northern type, i.e., grinders with two grindstones mounted on a common motor shaft, that the two grindstones be dressed alternately, so that the power consumption is kept fairly constant. If one grindstone approaches the end of its sharpness cycle, i.e., becomes dull, and hence draws a relatively large amount of power, the other one at least is only half worn and therefore draws less power. Therefore, it is of great importance, especially for establishing an automatic process control, that the operating personnel of a mill be able to measure, during operation, the sharpness of the grindstones and to formulate a plan as to when the worn grindstones should be dressed or sharpened.